1. Field of the Invention
The present invention relates to a linear amplifier, and more particularly, to a linear amplifier using a low frequency 2nd order intermodulation feed-forwarding method.
2. Description of the Related Art
When a mobile terminal or a mobile base station power-amplifies and transmits a signal, the receiving sensitivity of a receiving side is degraded by distortion due to the interference between a fundamental signal to be transmitted and its intermodulation component.
In order to solve the above problem, conventional linear amplifiers shown in FIGS. 1A, 1B and 2 are used. Each of the conventional linear amplifiers roughly includes an input impedance matching unit (Zs) 20 for matching the output impedance of a signal source Vs with the input impedance of the input port of a transistor 12, 14 or 24, and an output impedance matching unit (ZL) 29 impedance-matching the output port of the transistor 12, 14 or 24 with an output load. An amplifier shown in FIG. 1A tunes to a distortion component signal using a source harmonic tuning method and reflects an intermodulation signal reflected by the gate of the transistor 12 back to the gate using a reflection coefficient control unit 17 having a variable capacitor or a stub at the gate port of the transistor 12, thereby offsetting a distortion component. In FIG. 1B, a reflection coefficient control unit 16 is installed to the rear of the transistor 14, and tunes a harmonic output from the rear of the transistor 14 and reflects the harmonic back to the gate using a load harmonic tuning method, thereby offsetting a 3rd order harmonic component.
FIG. 2 shows a linear amplifier using an intermodulation signal feed-backwarding method. The linear amplifier of FIG. 2 includes an amplification transistor 24, filters 25 and 28 for filtering a 2nd order harmonic component from an input signal, a phase shifting unit 26 for shifting the phase of the 2nd order harmonic component, and an amplifier 27 for amplifying the phase-shifted 2nd order harmonic component to a predetermined value. The phase-shifted 2nd order intermodulation signal is fed backward to the input port of the amplification transistor 24, thereby offsetting a 3rd order intermodulation component.
However, these conventional intermodulation signal tuning methods can offset only several dB of 3rd order harmonic component. In the intermodulation signal feed-backwarding method, 10 dB to 15 dB of intermodulation component can be offset, but the characteristics is unstable and the band is narrowed.
An objective of the present invention is to provide a linear amplifier using a low frequency 2nd order intermodulation feed-forwarding method for offsetting a 3rd intermodulation component while maintaining the stable characteristics of a linear amplification circuit obtained by feed-forwarding.
To achieve the above objective, the present invention provides a linear amplifier using a low frequency 2nd intermodulation feed-forwarding method to offset the 3rd intermodulation component of a fundamental signal, the device including: a transistor for amplifying the power of the fundamental signal; an input impedance matching unit connected to the gate of the transistor for impedance-matching a source of the received fundamental signal with the gate of the transistor; a first filter unit, one end of which is connected to the gate of the transistor, for filtering a 2nd order intermodulation signal from the fundamental signal; a low frequency amplifier connected to the other end of the first filter unit, for amplifying the 2nd order intermodulation signal with a predetermined voltage gain; a second filter unit connected in series between the output port of the low frequency amplifier and the drain output port of the transistor, for filtering the amplified 2nd intermodulation signal and feeding the same to the output port of the transistor; and an output impedance matching unit connected to the output port of the transistor for impedance-matching the output port of the transistor with an external load.
The voltage gain (xcex2) of the low frequency amplifier   -                              gm          3                ·                  C          0                            gmd        ·                  C          1                      .  
The gm3 and gmd are the coefficients of terms vgs(t)3 and Vgs(t)vds(t), respectively, of Equation:
ids[vgs,vds]=gmxc2x7vgs(t)+gdxc2x7vgs(t)+gm2xc2x7
vgs(t)2+gmdxc2x7vgs(t)vds(t)+
gd2xc2x7vds(t)2+gm3xc2x7vgs(t)3+gm2dxc2x7
vgs(t)2xc2x7vds(t)+
gmd2xc2x7vgs(t)xc2x7vds(t)2+gd3xc2x7vds(t)3+. . . 
which is a Taylor series extension formula of the drain-source non-linear current ids[vgs,vds] of the transistor.
Also, C0 and C1 in the voltage gain are the coefficients of the constant term and term vgs(t) in Equation Cgs[vgs(t)]=C0+C1xc2x7vgs(t)+C2xc2x7vgs(t)2+ . . . , which is a power series of a gate source capacitor of the transistor.